1. Field of the Invention
The present invention relates to a bandwidth allocator, and more particularly to a bandwidth allocator for use in a telecommunications network such as a TDMA (Time Division Multiple Access) network. The present invention also relates to a method of allocating a bandwidth and a communication terminal therefor.
2. Description of the Background Art
With reference first to FIG. 15, a configuration of a general passive optical network (PON) will briefly be described which relies upon a TDM-PON (Time Division Multiplexing-PON) scheme as disclosed as a general background art in an article, “Technical Fundamentals Lecture [GE-PON Technology]”, NTT Technical Journal, pp. 71-74, August 2005.
A PON system 910 includes an optical line terminal (OLT) 920 located in a central office, a plurality of optical network units (ONUs) 930a, 930b, 930c, . . . located in subscriber premises, and an optical splitter 940 connecting the OLT 920 with the ONUs 930a, 930b, 930c, . . . . Communication, or transmission, from the ONU 930 toward the OLT 920 may be referred to as upstream communication, or transmission. Communication, or transmission, in the direction opposite thereto may be referred to as downstream communication, or transmission. The ONUs, generally 930, are connected with the optical splitter 940 by respective optical communication lines 970. The optical splitter 940 is connected with the OLT by another optical communication line 980.
The upstream communication in the TDM-PON system is based upon the TDMA scheme. In the TDMA scheme, the OLT 920 manages timing at which the ONUs 930 are allowed for transmitting signals such as to prevent the signals from colliding with each other on the optical communication line 980 connecting the OLT 920 to the optical splitter 940. Such control with the TDMA is disclosed by U.S. patent application publication No. 2007/0122151 to Watanabe.
Hereinafter, giving a permission by the OLT 920 to the ONUs 930 to transmit an upstream signal in a period of time having a predetermined length will be referred to as “allocating a bandwidth (or a time slot)”. A bandwidth allocated to the ONU 930 will be referred to as an “allocation grant”. Thus, the bandwidth and the allocation grant are represented in time length to be allowed for transmission, and therefore the units of the bandwidth and allocation grant are time, for example, second. Additionally, the volume of information which an ONU 930 can transmit during an allocation grant allocated thereto may be referred to as “the size of an allocation grant” or “an allocation grant size”. The unit of an allocation grant size is the amount of information such as byte.
FIG. 16 exemplarily shows how allocation grants are allocated to the ONUs by the OLT. In the figure, the ONUs 930a, 930b and 930c are allocated to allocation grants 950a, 950b and 950c, respectively. The sizes of the allocation grants 950a, 950b and 950c are indicated by m1, m2 and m3, respectively.
In the scheduling shown in FIG. 16, the ONU 930a transmits a signal during the period of the allocation grant 950a. Next, the ONU 930b transmits a signal during the period of the allocation grant 950b. Following the transmission from the ONU 930b, the ONU 930c transmits a signal during the period of the allocation grant 950c. In order to prevent those signals from collision, the allocation grants 950a, 950b and 950c do not overlap with one another.
Allocation grants may be allocated specifically to each of the ONUs 930, or dynamically to each of the ONUs 930 according to the amount of an upstream transmission requested by the ONU 930.
FIG. 17 shows how the ONU 930a uses the allocation grant 950a allocated thereto to transmit an upstream signal. FIG. 17, line (A), shows how a data buffer temporarily stores upstream data to be transmitted from the ONU 930a. The upstream data are in the form of packets P1, P2, P3 and P4 having variable length. Line (B) shows the allocation grant size m1 of the allocated allocation grant 950a. The allocation grant size m1 is smaller than the total size of the data packets P1, P2, P3 and P4 in the data buffer.
The ONU 930a uses this allocation grant 950a to transmit the data packets P1 to P4 in the data buffer. It is understood that when the data packets are transmitted sequentially in the order from the data packet P1, the allocation grant size m1 of the allocation grant 950a expires before completing the transmission of the last data packet P4. Then, the ONU 930a will transmit the data packet P4 next time.
In that case, actually the signal is transmitted as shown as FIG. 17, line (C), so that not all the allocated allocation grant 950a can be used up. A bandwidth, e.g. 985, successful in allocating an upstream data buffer is referred to as a used bandwidth, and an unused bandwidth, e.g. bandwidth 990, is referred to as allocation loss. When allocation loss is caused, the use efficiency of an upstream communication bandwidth decreases, which is problematic.